1. Field of the Invention
The present invention relates to an optical information recording medium such as an optical disc to record information on tracks, and more particularly to an optical multi-layer information recording medium having a plurality of recording layers stacked via light-transmissive spacer layer(s).
2. Description of the Related Art
In recent times, optical discs are widely utilized as means for recording and reproducing data such as image data, voice data and computer data. For example, DVD (Digital Versatile Disc) are marketed as optical discs that can record data at high density. Various types of DVDs are available with respect to schemes of data recordation and retrieval. One type of DVD is a multi-layer optical disc having a plurality of recording layers.
One example of a DVD is a read-only, two-layer optical disc. Referring to FIG. 1 of the accompanying drawings, the two-layer optical disc has a proximal recording layer (occasionally referred to as xe2x80x9cfirst layerxe2x80x9d), which is closer to an object lens of a pickup when viewed from a reading side, and a distal recording layer (occasionally referred to as xe2x80x9csecond layerxe2x80x9d) which is further from the object lens. The first layer is semitransparent so that it is possible to read a signal from the second layer through the first layer.
A spacer layer is provided between the first and second layers such that the first and second layers are equally spaced from each other. The spacer layer is light transmissive. Since a scanning beam passes through the spacer layer, the material of the spacer layer has transmissivity that is high at a wavelength of the reading beam.
It is possible to read signals from the first and second layers from the same side of the optical disc by only shifting a focus of the scanning beam if the two-layer optical disc is used. Shifting the focus is called focus jump or jumping. This type of two-layer optical disc is marketed as DVD-ROM (Read Only Memory) when the two-layer optical disc is designed for read-only purpose.
If the thickness of the spacer in the DVD-ROM two-layer optical disc is large, i.e., if the distance between the first and second layers is large, the beam of light defocuses and spreads at the second layer when the beam of light focuses on the first layer. As a result, pits and/or recordation marks in the second layer are not resolved, and a reflection light from the second layer is hardly modulated by the pits. Thus, if an average reflectance of the second layer does not change, it is feasible to read only the signal from the first layer by taking a high frequency component from the read signal with a high-pass filter. Likewise, if the scanning beam focuses on the second layer, it is possible to read only the signal of the second layer.
In order to increase the recording capacity, a next generation optical disc system has a pickup equipped with an object lens having a large numerical aperture (e.g., NA=0.85) and employs an optical disc having a larger number of stacked layers than a current DVD.
When the numerical aperture of the object lens increases, spherical aberration occurs if the thickness of the light-transmissive cover layer at the outermost surface of the optical disc is deviated from a standard value since a scanning or recording beam passes through the cover layer. As a result, a spatial frequency characteristic (MTF) which the object lens inherently possesses is not obtained. In particular, when a short mark or pit is recorded or read, irregularity in the cover layer thickness results in increased jitter. Because the multi-layer optical disc has a cover layer and a large number of spacer layers, a total thickness of layers from the cover layer to a target recording layer is large, and therefore the spherical aberration should be suppressed.
In order to solve the above described problem, an expander (optical unit including a combination of two lenses) is located between a light source and an object lens to compensate for the spherical aberration. The expander causes an incident parallel beam to become a convergent or divergent beam and to emerge towards the object lens. Consequently, the emergent beam from the object lens has certain spherical aberration that compensates for the spherical aberration produced at the cover layer. When the cover layer thickness has a standard value, the expander causes the parallel incident beam to emerge as the parallel beam so as to slightly change a diameter of the beam. If the object lens is designed such that spherical aberration generated when the beam emerges from the object lens counterbalances spherical aberration generated at the cover layer having the standard thickness, a light spot focused through the cover layer has no spherical aberration. If, on the other hand, the cover layer thickness deviates from the standard value, the expander causes a divergent or convergent beam, rather than a parallel beam, to emerge. For example, when the cover layer is thinner than the standard thickness, the expander causes the convergent beam to emerge towards the object lens. The spherical aberration produced in the object lens is increased but counterbalanced by the spherical aberration that is reduced by the cover layer. Thus, no aberration occurs on an information recording surface. An appropriate correction is made.
When the above described correction is performed, however, the focusing manner of the incident beam is changed. In general, therefore, not only the spherical aberration but also the numerical aperture of the object lens vary. For instance, when the converged beam is caused to be incident to the object lens, the emergent beam of the object lens focuses at a position before an originally designed focusing position of the object lens, and the numerical aperture increases. Accordingly, the numerical aperture varies with the correction made on the spherical aberration. If the numerical aperture changes together with the spherical aberration correction, the spot diameter on the recording layer changes with an amount of spherical aberration correction, i.e., position of the recording layer. As a result, a problem is created in that an optimum recording power varies when a signal is recorded. Compensation by a reproduction signal equalizer or the like is not effective during the recording so that perfect characteristic compensation has to be made before the recording.
In order to solve the above described problems, a recording apparatus may change the recording power in accordance with the spot diameter that changes between the first and second layers. When the recording power should be changed, however, the recording apparatus has to perform complicated power control and/or have a complicated structure if high speed modulation is performed in the recording power control because the power control is needed for the first and second layers respectively. Particularly, when high density recording is required, the recording power should be changed and the recording pulse waveform should be adjusted.
An object of the present invention is to provide an optical multi-layer information recording medium that can maintain power density of a spot on an optical disc to be substantially constant even if a numerical aperture change results from compensation made to correct spherical aberration at different recording layer positions, whereby recording characteristic is maintained even if recordation is performed without adjusting the recording power.
According to one aspect of the present invention, there is provided an optical multi-layer information recording medium for recording information therein and/or retrieving information therefrom upon radiation of a convergent beam of light, comprising at least two pairs of recording layers and light-transmissive layers stacked one after another, wherein a first recording layer proximal to a beam radiation side and a second recording layer distal from the beam radiation side are made from a material that satisfies the following equation:
(Trxxc3x97Trzxc3x97Aby)/Abx≈(NAy)2/(NAx)2
where Trx represents transmissivity of the first recording layer, Trz represents a sum of transmissivity of recording layer(s) and spacer layer(s) stacked between the first and second recording layers, Aby represents absorptance of the second recording layer, Abx represents absorptance of the first recording layer, NAy represents a numerical aperture on the second recording layer and NAx represents a numerical aperture of the first recording layer.
At least one of the recording layers may include a land track and/or a groove track. At least one of the recording layers may be made from a phase change material. The recording layers may be fabricated from different materials.
According to another aspect of the present invention, there is provided an optical multi-layer information recording medium for recording information therein and/or retrieving information therefrom upon radiation of a convergent beam of light, comprising a first recording layer proximal to a beam radiation side, a light-transmissive layer stacked on the first recording layer, and a second recording layer stacked on the light-transmissive layer and distal from the beam radiation side, wherein the first and second recording layers are made from a material that satisfies the following equation:
(Tr1xc3x97Ab2)/Ab1≈(NA2)2/(NA1)2
where Tr1 represents transmissivity of the first recording layer, Ab2 represents absorptance of the second recording layer, Ab1 represents absorptance of the first recording layer, NA2 represents a numerical aperture on the second recording layer and NA1 represents a numerical aperture on the first recording layer.
At least one of the first and second recording layers may include a land track and/or a groove track. At least one of the first and second recording layers may be made from a phase change material. The first and second layers may be fabricated from different materials.